This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A108351 #8 Nov 24 2017 06:27:28
%S A108351 1,1,2,3,5,8,14,24,43,77,140,255,467,856,1572,2888,5309,9761,17950,
%T A108351 33011,60713,111664,205378,377744,694775,1277885,2350392,4323039,
%U A108351 7951303,14624720,26899048,49475056,90998809,167372897,307846746
%N A108351 Diagonal sums of symmetric triangle A108350.
%H A108351 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,-2,-1,0,1).
%F A108351 G.f.: (1-x-x^2)/((1+x)(1-x)^2(1-x-x^2-x^3)); a(n)=2a(n-1)+a(n-2)-2a(n-3)-a(n-4)+a(n-6); a(n)=sum{k-0..floor(n/2), sum{j=0..n-2k, C(k, j)C(n-k-j, k)*(1+(-1)^j)/2}}.
%t A108351 LinearRecurrence[{2,1,-2,-1,0,1},{1,1,2,3,5,8},40] (* _Harvey P. Dale_, Nov 24 2017 *)
%K A108351 easy,nonn
%O A108351 0,3
%A A108351 _Paul Barry_, May 31 2005